Fibre Channel standards define protocols for link-level and end-to-end congestion control. However, these standard protocols do not eliminate head of line (HOL) blocking within a switch. HOL blocking is a problem for internal switching that occurs when several packets at the head of an input queue block packets from being forwarded to output ports. Storage Area Network (SAN) switches that share egress buffer resources are particularly susceptible to HOL blocking when they become congested because, unlike typical TCP/IP switches and routers, a SAN switch does not discard traffic when it becomes congested.
The Fibre Channel link-level flow control mechanism (buffer to buffer credits or BB Credits) is typically provided on a per-link basis to devices attached to the SAN switch. Occasionally, several ingress ports may share a pool of BB credits to receive traffic. In most SAN switches, egress congestion is not communicated to the ingress ports to limit the amount of ingress traffic. As a result, HOL blocking may occur within the switch as pools of shared memory become congested. Another major problem with buffer-to-buffer flow control model is that it is difficult to determine the number of BB Credits that are needed to efficiently move the frames. This is critical because the system needs enough credits to be able to provide a continuous flow between ports.